13th International Conference on Fracture June 16–21, 2013, Beijing, China 7 5Calculation of The CTOD test assessment point 5.1Abscissa of the assessment points In the Level-2B assessment, the horizontal axis is Lr, which could be calculated with the the equation (7) as mentioned above in section 4 with the Standard BS7910: While the CTOD theory was put forward initially, it was recognized that, for the elastic-perfectly plastic material, there was a plasticity area in the crack tip. Which means that the stress of the ligament area in the crack tip would not increase, after the crack tip fracture stress under general yielding. The stress of the plastic yielding area was equal to the yield strength σy .Therefore, while assessed the CTOD values with the Level-2B method, the value of Lr should be taken as 1.0 [10]and the abscissa of the assessment point should be 1.0 as well. 5.2 Ordinates of the assessment points In the FAD, the vertical coordinate was fracture ratio δr, determined by the following equation: r = / mat δ δ δ Ι (8) Where δmat is the fracture ratio of the material obtained by CTOD test, and the δI are calculated by KI as the following equation: 2 ' Y K = E X δ σ Ι Ι (9) X is the factor(generally of value between 1 and 2) influenced by crack tip and geometric constraint and the work hardening capability of the material. Appropriate values of X may be determined from elastic analyses which model structural constraint. If values of X are not quantified by structural analyses, use X =1.0. E' is the elastic modulus considering restrain. In the plane stress condition, ' E=E, while ' 2 E=1- μ, the in the plane strain condition. The applied stress intensity factor of the I-type crack, KI, has the following general form in the three bending condition: ( ) 0 3/ 2 W a f BW FS K ⋅ = Ι （10） 3/2 0 0 2 0 0 0 0 1/2 0 0 2 1 2 1 2.7 2.15 3.93 1 1.99 3 ⎟ ⎠ ⎞ ⎜ ⎝ ⎛ ⎟ − ⎠ ⎞ ⎜ ⎝ ⎛ + ⎥ ⎥ ⎦ ⎤ ⎢ ⎢ ⎣ ⎡ ⎟ ⎟ ⎠ ⎞ ⎜ ⎜ ⎝ ⎛ ⎟ ⎠ ⎞ ⎜ ⎝ ⎛ + ⎟ − ⎠ ⎞ ⎜ ⎝ ⎛ ⎟ − ⎠ ⎞ ⎜ ⎝ ⎛ − ⎟ ⎠ ⎞ ⎜ ⎝ ⎛ ⎟ = ⎠ ⎞ ⎜ ⎝ ⎛ W a W a W a W a W a W a W a W a f （11） where, S denotes the distance between specimens, W denotes the width and S =4W, B denotes the thickness, F denotes the loads. 5.3 Toughness assessment of CTOD
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